Pension and Other Postretirement Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	0 Months Ended				3 Months Ended	6 Months Ended	9 Months Ended				3 Months Ended		9 Months Ended			3 Months Ended
	Dec. 31, 2015 Scenario, Forecast [Member]	Dec. 31, 2014 Scenario, Forecast [Member]	Jun. 06, 2014 United Steelworkers Labor Agreement [Member]	May 31, 2014 United Steelworkers Labor Agreement [Member]	Sep. 30, 2014 United Steelworkers Labor Agreement [Member]	Jun. 30, 2014 United Steelworkers Labor Agreement [Member]	Sep. 30, 2014 United Steelworkers Labor Agreement [Member]	Jun. 06, 2014 United Steelworkers Labor Agreement [Member] Employees Location	May 31, 2014 United Steelworkers Labor Agreement [Member]	Dec. 31, 2013 United Steelworkers Labor Agreement [Member]	Dec. 31, 2014 United Steelworkers Labor Agreement [Member] Scenario, Forecast [Member]	Sep. 30, 2014 Postretirement Benefit Plan [Member]	Sep. 30, 2014 Postretirement Benefit Plan [Member]	Aug. 31, 2014 Postretirement Benefit Plan [Member]	Dec. 31, 2013 Postretirement Benefit Plan [Member]	Dec. 31, 2014 Postretirement Benefit Plan [Member] Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Term of labor agreement, in years			5 years
Number of employees covered by labor agreement								6,100
Number of locations affected by labor agreement								10
Labor agreement expired, date							May 15, 2014
Pretax costs related to preparation for and ratification of new agreement							$ 18
After-tax costs related to preparation for and ratification of new agreement							12
Discount rate used to determine pension benefit obligation									4.25%	4.80%				4.15%	4.80%
Increase in pension liability, plan amendment				100
Accumulated (increase) decrease in net actuarial loss after-tax, plan amendment						(65)							59
Annual decrease in net periodic benefit cost, plan amendment					6	13	8				5	2	7			5
Pension contributions minimum required	125	100
Decrease in other post benefit liability													$ 90